Works for old Reddit, I didn’t bother with new Reddit. Wonder what kind of kickbacks the mods are getting for this.
If you don’t know what this is: https://www.tampermonkey.net/ Install it, click the icon, ‘Create a new script…’ and paste this there.
// ==UserScript==
// @name pcgaming ad replacement
// @version 1.0
// @grant none
// @include https://reddit.com/r/pcgaming*
// @include https://*.reddit.com/r/pcgaming*
// ==/UserScript==
(function () {
const header = document.querySelector('#header');
header.style.background = 'none';
header.style.backgroundColor = 'black';
})();